In recent years, chemical vapor deposition (CVD) diamond coatings have been applied to a variety of substrate-material cutting tools intended for the same applications as single point, brazed-on polycrystalline diamond (PCD) tipped tools (see "Advanced Cutting Tool Materials," Kennametal Inc. (1988), Pages 1, 2, 77-86, 94-98, 101 and 102). While CVD diamond coated tools provide the machinist with multiple cutting edges on inserts with or without chipbreaker structures, their inconsistent machining results, due to poor coating adhesion, has resulted in a failure of the CVD diamond coated tools to be competitive with PCD tools in most commercial applications.
Various approaches have been made to the formation of diamond coating layers on various surfaces by CVD methods (e.g., hot filament, DC plasma Jet and microwave plasma) in which gases such as methane (CH.sub.4) are thermally decomposed. However, diamond coating layers formed by low pressure vapor-phase synthesis methods generally have a low adhesive bond strength to the substrate. Accordingly, what is desired is a coated substrate in which the adherence of the coating to the substrate is sufficient to retain the coating on the substrate for the time that it takes for the coating to gradually wear out by abrasion during machining of a workpiece material. Early or premature flaking of the coating prior to the wearing out of the coating causes unpredictable and inconsistent tool lifetimes, which is unacceptable to most users of PCD tipped tools. In addition, the diamond coating thickness should be thick enough so that each cutting edge provides at least forty percent of the wear life of PCD tools in order to be competitive with those tools.
One approach to this problem is disclosed in U.S. Pat. No. 5,068,148, which issued on Nov. 26, 1991. The '148 reference discloses a method for producing a diamond coated tool member wherein a cemented carbide substrate is chemically etched to remove cobalt existing in the outermost portion of the substrate. Such etching steps may generate internal interconnecting porosity which diminishes the toughness and wear resistance of the cutting tool insert, but absent chemical etching, tool performance may diminish due to coating delamination caused by poor preparation of the substrate surface (e.g., too much cobalt left on the surface). The '148 reference calls for heat-treating a ground substrate at a temperature between 1000.degree. C.-1600.degree. C. for 30 to 90 minutes in a vacuum or in a non-oxidizing atmosphere before chemical etching. If the heat treating temperature exceeds 1600.degree. C., the hard grains of the substrate become bulky, and the surface of the substrate becomes extraordinarily rough, so that the substrate cannot be used for manufacturing a tool member.
In another approach, disclosed in European Patent Application No. 0 518 587, the surfaces of a cemented tungsten carbide substrate are also etched for the purpose of improving diamond coating adherence.
It is the inventors' belief, after examination of diamond coated cemented carbide tools presently being commercially marketed, that where an etching step is used to improve diamond adhesion (to 60 to 100 kg in the Rockwell A indentation adhesion test), etching has preferentially removed significant amounts of cobalt from the surface and from just beneath the surface. This results in interconnected porosity just beneath the substrate surface, creating a weakened structure which undermines the ability of the diamond coating to remain attached to the tool during machining operations, and which results in flaking of the coating, especially during interrupted machining operations.
U.S. Pat. No. 5,204,167, which issued on Apr. 20, 1993, discloses a diamond coated sintered body in which the average size of recrystallized tungsten carbide in the surface layer is finer as compared with that existing in the inner portions of the substrate. The '167 reference teaches that increased adhesion between the diamond film and the substrate is because graphite generated at an initial stage of diamond deposition is used for recarburization of a surface decarburized layer of the substrate, so that graphite formed at the interface between the surface layer and the film is decreased.
Such approaches leave unsolved the challenge of providing a high bond strength between the coating and the substrate.
Current practice in the design of conventional, PCD cutting tools calls for the tool to have a sharp cutting edge for both turning and milling applications on non-ferrous and non-metallic workpieces. The use of sharp edges provides lower cutting tool forces during machining and workpiece surface finishes having the required characteristics, e.g., low surface roughness.
Diamond coated cutting tool inserts should ideally provide the same workpiece surface characteristics to be commercially competitive with conventional PCD tools. Another one of the factors currently limiting the acceptance of diamond coated tools has been the difficulty of providing acceptable workpiece surface finishes, especially in finishing operations. Conventional PCD tools often contain a metallic binder, such as cobalt, which holds the diamond particles together. When properly ground, the PCD provides a substantially smooth cutting surface and imparts a substantially smooth surface to the workpiece. In contrast, diamond coatings do not contain a binder phase. They typically have a rough, faceted surface on a microscopic scale. Such microscopic roughness leads to rough workpiece finishes in cutting operations. Under prior approaches, the purer (or more perfect) the diamond coating, i.e., more sp.sup.3 and less sp.sup.2 (graphitic) bonded component, the more highly faceted the coating becomes. Such coatings can be made smoother by increasing the amount of graphitic component, but wear resistance, and tool lifetime, decrease as a result. Although chemical polishing with reactive materials and compounds or mechanical polishing with diamond grit may be used to produce a smooth diamond surface, the road remained open for improved approaches.
Accordingly, it would be desirable to provide a high purity diamond coating on a cutting tool substrate that will be highly adherent in use, and will preferably achieve workpiece surface finishes comparable to those provided by conventional PCD tools.
Until the present invention, there remained an unsolved need for simple, yet effective techniques for consistently providing a highly adherent diamond coating and for providing a smooth surface of a high purity, highly faceted diamond coating on a three-dimensional shape, e.g., a cutting tool insert.